1. Field of the Invention
This invention relates to vitrification of organic ion exchange resins into borosilicate glass, in particular into iron-enriched borosilicate glass, by adding borosilicate glass formers and a ferric oxide producer directly to the resins, forming a homogeneous and durable waste form. The invention results in significant volume reductions of the ion exchange resins.
2. Description of the Related Art
The commercial nuclear industry utilizes ion exchange resins to clarify their process and storage waters. The resins, which typically contain one or more backbone polymers and one or more functional groups, are used to remove unwanted impurities, such as radioactive materials or other contaminants, that could potentially harm the equipment or corrode reactor fuel rods. The resins can clarify water which is to be reused at or discharged from the plant, or that is to be stored on site. Often, significant quantities of liquids are treated in this way, creating large volumes of waste solutions. Ion exchange resins are used in several processes to remove both hazardous and radioactive constituents from these solutions or sludges, making disposal of the solutions or sludges easier. In reactor facilities, ion exchange resins are typically used for purification of water in reactor basins and fuel storage basins.
Over time, these resins have to be re-generated or replaced because there is an upper limit on the amount of material the resins can remove before they become fully loaded and ineffective. When this happens, the spent resins themselves become hazardous and/or radioactive waste requiring disposal. In many cases, the spent resins present disposal problems both because of the organic matrix itself and the radioactive and sometimes hazardous contaminants adsorbed thereon. Some of the radioactive contaminants that can be present include Cs.sup.37, Sr.sup.90, Co.sup.60, C.sup.14, Mn.sup.54 and Tc.sup.99. In the United States, resin wastes from Boiling Water Reactors (BWR) are enriched in constituents such as Fe.sub.3 O.sub.4, while wastes from Pressurized Water Reactors (PWR) are enriched in borate from moderators and in Li, from pH control compounds. Approximately 100,000 lbs. of BWR and 30,000 lbs. of PWR spent resins are generated per year per commercial reactor in the United States.
Resins for removing Cs from Department of Energy (DOE) high level waste (HLW) are being investigated by several DOE sites. A resorcinol resin was originally proposed for use in removing the Cs from HLW supernate. If the use of these resins is implemented, a disposal method suitable for several thousand pounds of spent resin will be needed. Divinylbenzene/styrene copolymer resins are used by reactor facilities, including those at the Savannah River Site (SRS), to purify fuel rod storage basin water.
The widespread use of ion exchange resins in the nuclear industry, which shows every sign of continuing into the future, has resulted in a need for a cost-effective method for disposing of spent resins. Disposal methods can be analyzed into two subparts: volume reduction and immobilization.
Various methods exist in the art for reducing the volume of these resins and for immobilizing them. U.S. Pat. No. 4,671,898 discloses converting a spent, radioactive ion exchange resin into a stable cement product having reduced volume. U.S. Pat. No. 4,632,778 discloses a procedure for transforming radioactive waste bound to an inorganic ion exchanger, yielding a ceramized product. U.S. Pat. Nos. 4,793,947 and 5,288,435 disclose vitrifications of radioactive waste products requiring pretreatment prior to vitrification.
Vitrification has been shown to be a feasible treatment method for ion exchange resins. The organic compounds which make-up the matrix of the resins can be destroyed either by pyrolysis or combustion at typical vitrification temperatures. Some of the heavier organic compounds are pyrolyzed within the melt. The majority of the combustion usually occurs above the melt in the plenum or in a secondary combustion chamber. The goal is to reduce the volume of the total waste, while at the same time providing a durable, immobilizing medium for the radioactive and/or hazardous species.
An independent study performed by the Electric Power Research Institute (EPRI) determined that a significant return on investment capabilities was possible by applying vitrification technology to the treatment of spent ion exchange resin. Another important determination was that implementation of the technology would give insurance to reactor operators that operations could continue even if regional compacts for low level waste disposal were delayed.
However, vitrification of organic ion exchange resins presents a challenge because of the high organic content of the resins and the volatile Cs.sup.137 that is usually present. High organics tend to induce reducing environments in melters, which can result in the reduction of metals in the waste, and separation of the metals from the bulk glass matrix, defeating at least one of the goals of vitrification. Alloying of the reduced metals with the melter electrodes or corrosion of other melter components can also be a problem, reducing the useful life of the treatment equipment. Organics can also result in reduced glasses, which have been shown to have poorer durability compared to glasses of the same composition that are oxidized or less reduced. X. Feng, I.L. Pegg, E. Saad, S. Cucinell, and A. A. Barkatt, "Redox Effects on the Durability and Viscosity of Nuclear Waste Glasses", Nuclear Waste Management IV, 23. Cs.sup.137 has been shown to be extremely volatile at high temperatures. Stabilization of this contaminant in the glass matrix without excessive volatilization presents a challenge which must be met if the waste is to be successfully stabilized.
Past attempts to vitrify ion exchange materials have been only moderately successful from the standpoints of waste loading and volume reduction, and have required additional pretreatment steps. Without pre-treatment, waste loadings and volume reductions have been very limited. A general maximum guideline for waste loading in the glass industry is approximately 20 weight percent. At this waste loading, final wasteform volume usually increases. Using pre-treatment methods (such as wet acid oxidation disclosed in U.S. Ser. No. 08/861,483, filed May 22, 1997 the entire contents of which are hereby incorporated by reference), these processes can result in volume reductions when the resin structure is broken down. The disadvantage of pre-treatment is that extra processing steps and equipment are required. Thus, there are more up-front capital costs and everyday supply costs. These costs are multiplied by the need to use equipment and procedures suitable for handling radioactive material. In addition, the requirement for pre-treatment would extend the treatment time required. Most pre-treatment steps involve some form of chemical oxidation or treatment, which will require control of the radioactive and hazardous materials associated with the resin (e.g. creating a Cs volatility concern). As a result, there exists a need for a method of directly vitrifying organic ion exchange resins in order to reduce the volume of the resin waste and produce a durable and stable waste form. It is one object of the present invention to provide such a process.
More specifically, it is an object of the present invention to provide a process for vitrifying an organic ion exchange resin without pretreatment of the resin.
It is another object of the invention to provide a process for converting organic ion exchange resins into homogeneous and durable waste forms of iron-enriched borosilicate glass. More specifically, it is an object of the present invention to vitrify these resins directly by adding borosilicate glass formers and a ferric oxide producer to aid in oxidation reactions that remove organic materials during melting.
It is another object of the invention to provide a process for converting organic ion exchange resins into homogeneous and durable waste forms of iron-enriched borosilicate glass by adding ferric nitrate as a ferric oxide producer, where the ferric nitrate provides nitrates to help oxidize the organic materials.